Natural killer (NK) cells and K cells mediating antibody-dependent cellular cytotoxicity have been shown to be large granular lymphocytes (LGL). The majority of LGL form lytic conjugates with a wide variety of NK-susceptible target cells. The relationship of soluble factors to this cell mediated lysis, has been examined using NK cytotoxic factors (NKCF) as the mechanism of action. Three distinct steps have been defined for NKCF: a) production, b) binding to targets, and c) subsequent target lysis. Using rat LGL cell lines as a source for NKCF, we have begun its molecular cloning with the aid of the specific monoclonal antibody to NKCF. Since, the anti-NKCF MAb's were prepared against purified cytolysin, we examined their reactivity against purified granule- derived cytolysin. The data clearly demonstrated an inhibition of lysis by purified cytolysin, but only after Ca++ events. In addition, since nuclear degradation has been implied in cellular lysis, NKCF was examined for this property. NKCF was capable of mediating rapid nuclear degradation of YAC, which was abrogated with the anti-NKCF MAb. In addition to lytic mechanisms, studies are proceeding to define the receptors and structures involved in NK recognition. An anti-idiotypic antibody (anti-ID) has also been developed against a target monoclonal antibody capable of blocking LGL binding, anticipating that this antibody would recognize the NK receptor and aid in its identification. This anti-ID antibody is reactive with a 85 kD protein and blocks LGL binding and target cell lysis. Finally, when LGL were pretreated for 18 hrs with the F(ab')2 anti-ID antibody, there was significant enhancement of lysis and production of IFN-gamma. It will be important in future studies to characterize the expression of the 85 kD molecule on activated cells that have reported NK activity. Further biochemical characterization of the NK-R and its cloning are anticipated in future studies.